According to the European standardization body for 3G mobile telephony, the downlink and spreading modulation has been described in 3GPP TS 25.213 V6.2.0 chapter 5.
FIG. 1 is a reproduction from the above document showing the spreading and modulation principle. In SP, a downlink physical binary bit stream is serial to parallel converted in S/P stage and delivered to modulation and phase mapper, which again produce a mapping into the IQ plane of e.g. 16 QAM symbols. The symbols are spread by the high frequency mobile terminal specific channelization code Cch,SF,m and by a cell specific scrambling code Sdl, n, both signals having a duration of one chip.
A base station unit must at least perform downlink transmission relating to a number of bit streams on a plurality of physical channels (e.g. 512 channels), relating for instance to individual mobile users. The information on the downlink channels are spread and modulated according to the above method and added such that one output signal is amplified and transmitted by the base station antenna.
One approach for performing this task is to provide modulation and spreading resources per physical channel, whereby each modulation and spreading resource performs spreading of the bit stream pertaining to the channel in question.